1. Field of the Invention
The present invention relates to an optical power monitor for measuring the intensity or energy of light in an optical fiber.
2. Description of the Related Art
With the rapid proliferation of the Internet, remarkable technical and quantitative developments have been made in the optical fiber communication market and optical communication networks are still presently being increased. With the increase in use of wavelength multiplex communication systems as one of large-capacity communication means, a need has arisen to handle several ten to several hundred wavelengths per system. There is a need to measure and monitor each of the energy of light of different wavelengths in operating a wavelength multiplex communication system. An optical power monitor is used for this purpose. A multiplicity of optical power monitors are used in a wavelength multiplex communication system. There is, therefore., a demand for reducing the size and price of optical power monitors.
In the optical power monitor most frequently used presently, an optical signal coming out of an optical fiber is received by a photo diode to be taken out as an electrical signal.
In an optical power monitor shown in section in FIG. 5A, a pigtail fiber 32 is used which is assembled by inserting an optical fiber 35 in a through-hole formed in a columnar capillary 34 along the center axis of the columnar capillary 34 so that the center axis of the columnar capillary 34 and the center axis of the optical fiber 35 coincide with each other. The pigtail fiber 32 and a photo diode 3 with a lens are fixed coaxially with each other in a cylindrical tube 6 so that a light emission side end surface 37 of the pigtail fiber 32 and the lens provided on the photo diode 3 are opposed to each other through a certain spacing set therebetween. As indicated by the arrow in the figure, an optical signal emitted from an open end surface of the optical fiber 35 into the space between the optical fiber open end surface and the lens provided on the photo diode 3 enters a light receiving portion 9 via the vertex 8 of the lens provided on the photo diode 3 to be converted into an electrical signal. In this optical power monitor, the open end surface of the optical fiber 35 is perpendicular to the center axis of the optical fiber 35. Therefore, part of the optical signal is reflected by the open end surface of the optical fiber 35 to travel in the reverse direction in the optical fiber and interfere with the optical signal traveling forward, thus causing reflection loss.
To reduce reflection loss, a method has been practiced in which, as described in Japanese Patent Laid-Open No. 2001-13362 or in the published Japanese translation No. 10-511476 of a PCT application, the light emission side end surface of a pigtail fiber is inclined through a certain angle (e.g., about 4 to 10 degrees) from the center axis of the pigtail fiber to have this angle with respect to the center axis. FIG. 5B shows in section an optical power monitor in which a light emission side end surface 47 of a pigtail fiber is set at a certain angle with respect to the center axis of the pigtail fiber. The reflection loss of an optical signal can be reduced setting the open end surface of the optical fiber at a certain angle from the center axis of the optical fiber. In the case where the optical fiber open end surface is inclined with respect to the center axis, however, the optical signal radiated from the optical fiber open end surface into the space between the optical fiber open end surface and the lens is bent through a certain angle with respect to the center axis of the optical fiber. Accordingly, the radiated optical signal enters the light receiving portion 9 by traveling via a position deviating from the lens vertex 8. When the optical signal enters the light receiving portion 9 via a position deviating from the lens vertex 8, the signal output from the light receiving portion 9 is reduced and the linearity of the sensitivity of the light receiving portion 9 is also reduced. If strong light strikes an end of the light receiving portion 9, a reverse current is generated and the sensitivity is further reduced.
A method using an arrangement in which as shown in section in FIG. 5C the center axis of a pigtail fiber 52 and the center axis of a photo diode 3 are shifted from each other to enable light radiated from the optical fiber open end surface to enter a light receiving portion 9 by traveling via a lens vertex 8 has been practiced. Shifting the center axes in such a manner requires using a cylindrical tube 56 having an inside diameter larger than that of the cylindrical tube 6 shown in FIG. 5A or 5B. Positioning of the pigtail fiber 52 and the photo diode 3 at the time of fixing by bonding in the tube 56 having an inside diameter sufficiently larger than the outside diameters of the pigtail fiber and the photo diode is considerably difficult to perform.
As a structure in which a pigtail fiber and a photo diode can be easily bonded and fixed while shifting the center axes of the pigtail fiber and the photo diode, a pigtail-type optical module in which the center axis of a pigtail fiber and the position of an optical element are shifted from each other is disclosed in Japanese Patent Laid-Open No. 5-34370. FIG. 6 is a sectional view of the pigtail-type optical module 60 disclosed in this document. A pigtail fiber 67 is bonded and fixed in a generally cylindrical sleeve 62, with their center axes aligned with each other. An optical element 68 having a spherical lens 63 is bonded and fixed in a generally cylindrical holder 61, with their center axes aligned with each other. The center axis of sleeve 62 is fixed eccentrically to the center axis of the holder 61 so that light coming out of the optical element 68 via the lens vertex 69 of the spherical lens 63 travels along the center axis of the pigtail fiber 67. A ferrule 66 (also called a capillary) is coaxial with an optical fiber 65.
The structure of the pigtail-type optical module 60 shown in FIG. 6 will be briefly described. The construction of the pigtail-type optical module 60 shown in FIG. 6 is the same as that of each of the optical power monitors shown in FIGS. 5A to 5C, although the direction of travel of light is reverse to that in the optical power monitors shown in FIGS. 5A to 5C. Light coming out of a light emitting element 64 passes through the spherical lens 63 and enters the optical fiber 65. The light emitting element 64 is provided eccentrically to the center axis of the optical element 68 to collect light coming out of the light emitting element 64 on the lens vertex 69 of the spherical lens 63 existing on the center axis of the optical element 68. To introduce light coming out of the lens vertex 69 into the optical fiber 65, the sleeve 62 in which the pigtail fiber is held and the holder 61 are fixed at an optimum relative position after shifting the sleeve 62 along the X-direction. There is also a need to adjust the pigtail fiber 67 along the Z-direction and along the φ-direction with respect to the optical element 68.
The structure shown in FIG. 6 ensures that light coming out of the spherical lens 63 can be led to the optical fiber 65 with reliability. If this configuration is applied to an optical power monitor, light coming out of an optical fiber can be led to a lens vertex with reliability. Positioning is remarkably easier in the structure shown in FIG. 6 than in the optical power monitor shown in FIG. 5C. The structure shown in FIG. 6, however, requires preparing two kinds of sleeve, i.e., the sleeve 62 and the holder 61. This means difficulty in reducing the size and cost. Moreover, it is necessary to adjust the pigtail fiber 67 along the X—, Z—, and φ-directions with respect to the holder 61.